Hydrogen separation membrane on a porous substrate

ABSTRACT

A hydrogen permeable membrane is disclosed. The membrane is prepared by forming a mixture of metal oxide powder and ceramic oxide powder and a pore former into an article. The article is dried at elevated temperatures and then sintered in a reducing atmosphere to provide a dense hydrogen permeable portion near the surface of the sintered mixture. The dense hydrogen permeable portion has a higher initial concentration of metal than the remainder of the sintered mixture and is present in the range of from about 20 to about 80 percent by volume of the dense hydrogen permeable portion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/418,957, filed on May 5, 2006, which claims the benefit of U.S.Provisional Patent Application Ser. No. 60/711,962 filed on Aug. 25,2005 and U.S. Provisional Patent Application Ser. No. 60/711,961 filedon Aug. 25, 2005, each of which is incorporated herein by reference inits entirety.

CONTRACTUAL ORIGIN OF THE INVENTION

The United States Government has rights in this invention pursuant toContract No. W-31-109-ENG-38 between the U.S. Department of Energy andThe University of Chicago representing Argonne National Laboratory.

FIELD OF THE INVENTION

This invention relates to a membrane and method for extracting hydrogenfrom fluids and, more particularly, this invention relates to a newmethod of making a high-flow rate membrane and an improved method forextracting hydrogen from fluid without using electrical power orcircuitry.

BACKGROUND OF THE INVENTION

Global environmental concerns have ignited research to develop energygeneration technologies which have minimal ecological damage. Concernsof global climate change are driving nations to develop electric powergeneration technologies and transportation technologies which reducecarbon dioxide emissions.

Hydrogen is considered the fuel of choice for both the electric powerand transportation industries. While it is likely that renewable energysources will ultimately be used to generate hydrogen, fossil-basedtechnologies will be utilized to generate hydrogen in the near future.

The need to generate ever larger amounts of hydrogen is clear. Outsideof direct coal liquefaction, other major industrial activities, such aspetroleum refining, also require hydrogen. Collectively, petroleumrefining and the production of ammonia and methanol consumeapproximately 95 percent of all deliberately manufactured hydrogen inthe United States. As crude oil quality deteriorates, and as morestringent restrictions on sulfur, nitrogen and aromatics are imposed,the need for more hydrogen by the refining industry will increase.

Hydrogen production, as a consequence of other processes, issignificant. A number of industries requiring hydrogen produce effluentscontaining significant amounts of unused hydrogen. However, thishydrogen requires clean-up prior to re-use. Furthermore, hydrogen isproduced from the gasification of oil, methane, coal, and otherpetroleum-based materials, as well as biomass and other renewableresources. However, this hydrogen must be separated from othercombustion gases, namely carbon dioxide, in order to be of use.

Petroleum refineries currently use cryogenics, pressure swing adsorption(PSA), and membrane systems for hydrogen recovery. However, each ofthese technologies has their limitations. For example, because of itshigh costs, cryogenics generally can be used only in large-scalefacilities which can accommodate liquid hydrocarbon recovery.Membrane-based PSA systems require large pressure differentials acrossmembranes during hydrogen diffusion. This calls for initial compressionof the feed prior to contact to the upstream side of polymeric membranesand recompression of the permeate to facilitate final purificationsteps. Not only are these compression steps expensive, but PSA recoversless feedstream hydrogen and is limited to modest temperatures. U.S.Pat. No. 5,447,559 to Rao discloses a multi-phase (i.e. heterogenous)membrane system used in conjunction with PSA sweep gases.

The subject invention is an improvement of the '226 membranes providingan easier method of fabrication of the composite membranes.

SUMMARY OF THE INVENTION

The object of this invention is to provide dense composite metal andceramic membranes that can nongalvanically separate hydrogen from othergaseous components and is an improvement to the membranes and methodsdisclosed in U.S. Pat. No. 6,569,226, the entire disclosure of which isincorporated by reference.

It is a principal object of the present invention to provide ahydrogen-separation membrane and an improved method of making same.

Another general object of the invention is to provide a membrane toextract hydrogen from a variety of fluids in which the membrane is madeby a thermal process hereinafter described.

Another object of the invention is to provide a method of making ahydrogen permeable composition, comprising forming a mixture of metaloxide powder and ceramic oxide powder and optionally a pore former,pressing the mixture to form an article, drying the article at elevatedtemperatures to remove at least some of the pore former if present andthereafter sintering in a reducing atmosphere to provide a densehydrogen permeable portion near the surface of the sintered mixture, thedense hydrogen permeable portion having a higher initial concentrationof metal than the remainder of the sintered mixture, the metal oxidebeing selected from the oxides of Ni, Pd, Pd alloys, Nb, Ta, Zr, V ormixtures thereof, the ceramic oxide part being selected from yttriastabilized zirconia, shrinkable alumina, suitably doped cerates,titanates, zirconates of barium or strontium or mixtures thereof,wherein the metal part is present in the range of from about 20 to about80 percent by volume of the dense hydrogen permeable portion.

A final object of the invention is to provide a method of making ahydrogen permeable composition, comprising forming a mixture of metaloxide powder and ceramic oxide powder and a pore former, pressing themixture to form an article, drying the article at elevated temperaturesto remove at least some of the pore former and thereafter sintering in ahydrogen-containing reducing atmosphere to provide a dense hydrogenpermeable portion not greater than about 50 microns in thickness and atleast about 96% of theoretical density near the surface of the sinteredmixture, the dense hydrogen permeable portion having a higher initialconcentration of metal than the remainder of the sintered mixture, themetal oxide being selected from the oxides of Ni, Pd, Pd alloys, Nb, Ta,Zr, V or mixtures thereof, the ceramic oxide part being selected fromyttria stabilized zirconia, shrinkable alumina, suitably doped cerates,titanates, zirconates of barium or strontium or mixtures thereof,wherein the metal part is present in the range of from about 20 to about80 percent by volume of the dense hydrogen permeable portion.

The invention consists of certain novel features and a combination ofparts hereinafter fully described, illustrated in the accompanyingdrawings, and particularly pointed out in the appended claims, it beingunderstood that various changes in the details may be made withoutdeparting from the spirit, or sacrificing any of the advantages of thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of facilitating an understanding of the invention, thereis illustrated in the accompanying drawings a preferred embodimentthereof, from an inspection of which, when considered in connection withthe following description, the invention, its construction andoperation, and many of its advantages should be readily understood andappreciated.

FIGS. 1( a) and (b) are SEM micrographs of dense ANL-1a film produced bythe inventive method; and

FIGS. 2( a)-(c) are SEMs of ANL 1a film reduced and sintered indifferent conditions.

DETAILED DESCRIPTION OF THE INVENTION

Argonne National Laboratory (ANL) is developing two types of novelceramic membranes for producing pure hydrogen: hydrogen transportmembranes (HTMs) and oxygen transport membranes (OTMs), see Table 1.Both types of membrane are dense and produce hydrogen nongalvanically,i.e., they require neither electrodes nor an external power supply. HTMsproduce hydrogen by separating it from mixed gases, e.g., productstreams generated during coal gasification and/or methane reforming,whereas OTMs generate hydrogen by removing oxygen that is producedduring the dissociation of water at moderate temperatures (<900° C.).

ANL Membrane Compositions Membrane Matrix Metal ANL-0 BCY — ANL-0bSFC(SrFeCo_(0.5)O_(x)) — ANL-0c SFT(Sr_(1.0)Fe_(0.9)Ti_(0.1)O_(x)) —ANL-1a BCY Ni ANL-lb CMO Ni ANL-lc TZ-8Y Ni ANL-1dSFT(Sr_(1.0)Fe_(0.9)Ti_(0.1)O_(x)) Ni ANL-2a BCY Pd ANL-2b CMO Pd/Ag(23wt. %) ANL-3a Al₂0₃ Pd ANL-3b BaTiO₃ Pd/Ag ANL-3c Al₂O₃ Nb ANL-3d Al₂O₃Pd/Ag(23 wt. %) ANL-3e TZ-3Y Pd ANL-3f TZ-8Y Pd ANL-3g CaZrO₃ Pd ANL-4aCu Nb Notes: BCY = BaCe_(0.8)Y_(0.2)O_(3−*) CMO = Ce_(1−x)M_(x)O_(2−*)(MGd, Y) TZ-3Y = ZrO₂ (3 mol. % Y₂O₃) TZ-8Y = ZrO₂ (8 mol. % Y₂O₃)

Because the hydrogen flux through ANL-3 HTMs appears to be limited bythe diffusion of hydrogen through the bulk, reducing the membranethickness is expected to increase the hydrogen flux, or allow the sameflux at lower temperatures. To increase the hydrogen flux through ANLmembranes and/or reduce their operating temperature, the inventivethermal process was used for fabricating dense thin film membranes.

For cermet hydrogen separation membranes with thickness greater thanabout 10 μm, the hydrogen flux is limited by the diffusion of hydrogenthrough the bulk. As a result, reducing the membrane thickness isexpected to increase the hydrogen flux or allow the same flux at lowertemperatures. Because very thin membranes are not strong, they needmechanical support from a relatively thick substrate. In order for gasesto flow to and from the membrane, the substrate must be porous, whilethe thin membrane must be dense to provide high selectivity forhydrogen. This invention relates to a simple, cost-effectivethermochemical method, for preparing a dense, thin film, cermet hydrogenseparation membrane on a porous substrate.

The thermochemical method has produced dense Ni/BCY thin films (about 25μm) on porous Ni/BCY, where BCY represents BaCe_(0.8)Y_(0.2)O_(3-δ). Inthis invention, the gas composition is carefully controlled duringsintering so that kinetic limitations confine the reduction of NiO tothe surface of a NiO/BCY compact. Because Ni/BCY densifies more readilythan NiO/BCY, only the surface layer densifies. The interior of thecompact remains NiO/BCY during sintering, and becomes porous when theNiO is subsequently reduced at a lower temperature. Because dense Ni/BCYthin films (or other combinations) can be produced without further wetchemical processing, the method is comparatively simple and is amenableto fabricating tubes with a dense Ni/BCY layer on either the interior orexterior surface.

To make ANL-1a thin films using the thermal method, BCY/NiO disks wereprepared by mixing NiO, BCY, and graphite (5 wt. %) powders, and thenuniaxially pressing the mixture into thin (1-2 mm) cylindrical disks (22mm diameter). The disks were pre-sintered at 700° C. for 5 hours in airto remove the graphite and provide the disks with some mechanicalintegrity and porosity. The disks were then sintered for 8-10 hours at1350-1425° C. in gases containing 200 ppm-4% H₂/balance N₂. Thecomposition of the disks was controlled to give 45 vol. % Ni and 55 vol.% BCY after elimination of the graphite and reduction of the NiO.

FIGS. 1( a) and (b) shows cross-sectional views of a BCY/NiO compactthat was sintered at 1400° C. for 8 hours in 200 ppm H₂/balance N₂. Thisfirst attempt to fabricate an ANL-1a thin film by the thermal methodproduced a dense layer with a thickness of ≈50 μm. Because the thicknessof the dense layer depends on the relative rates of sintering andNiO-reduction, it is controlled by parameters such as the sinteringtemperature, ramp rate to the sintering temperature, oxygen partialpressure during sintering, and porosity of the compact.

To study the parameters that affect the thickness of films made by thethermal method, ANL-1a films were made using three different heatingschedules. Fracture surfaces of the films are shown in FIG. 2. In allcases, the atmosphere was 200 ppm H₂/balance N₂ during sintering. A filmthickness of about 100 μm (FIG. 2 a) resulted when a sample was sinteredat 1420° C. without first being reduced in 4% H₂. Reducing a sample at1400° C. in 4% H₂/balance N₂ for 0.1 hour before it was sintered at1400° C. decreased the film thickness to 33 μm (FIG. 2 b), whereasreducing a sample at 500° C. in 4% H₂ for 0.1 hour before it wassintered at 1400° C. decreased the film thickness to 25 μm (FIG. 2 c).These results demonstrate that the film thickness can be manipulated byadjusting the conditions for heat-treating the BCY/NiO disks. Forsamples that were reduced before they were sintered, it was found thatthe film thickness increased with an increase in either the duration orthe temperature of the reduction step.

Porous substrates on which the powder mixtures can be deposited, ifdesired, can be made from either Al₂O₃ or a NiO/TZ-3Y mixture. Any ofthese ceramic oxide powders can be mixed with metal oxide powders toprovide a homogeneous mixture, if desired. Preferably, a homogeneousmixture of metal oxide and ceramic oxide powders are used to make thecermets, but the invention includes a porous ceramic substrate ontowhich is pressed a homogeneous mixture. Two types of Al₂O₃ powder weretested and eliminated from further consideration for related methods,because one powder densified completely during sintering, and the otherpowder did not shrink during sintering. Low shrinkage of the substrateduring sintering is a problem, because it hinders densification of thethin film. A third type of Al₂O₃ powder contained about 10 wt. % water,and experienced high shrinkage during sintering of the ANL-3e film thatwas deposited on it. The high shrinkage of the substrate was consideredbeneficial to densification of the thin film.

Sr—Fe—Co—O (SFC) powder for ANL-0b membranes has been purchased fromPraxair, whereas Sr—Fe—Ti—O (SFT) powder for ANL-1d membranes has beenprepared at ANL by conventional solid-state reaction between itsconstituent oxides.

Disk-shaped membranes were prepared by uniaxially pressing the powders,heating to temperatures below 1000° C. and then sintering the disks in avariety of reducing atmospheres, depending on the constituents used, andunder a variety of conditions of temperatures and times.

Hydrogen flux measurements during reduction of the substrate in acompanion application suggest that the effect of sintering temperatureon hydrogen flux might be related to the size of pores in the substrate.Because porosity increased during reduction improved hydrogen transportthrough the substrate, the hydrogen flux increased with time until thesubstrate was fully reduced, at which point the flux stopped increasing.Films sintered at 1500° C. were reduced in 3 hours, whereas the filmsintered at 1400° C. needed 12 hours to be reduced. Becauseinterconnected pores in the substrate aid reduction of NiO, the longertime to reduce the film sintered at 1400° C. suggests that it had eitherlower porosity, or smaller pores, than films sintered at 1500° C. Filmssintered at a lower temperature should not be less porous than filmssintered at a higher temperature, but sintering at a lower temperaturecould yield smaller grains of NiO, which would produce smaller poresafter the NiO was reduced. Smaller pores could impede hydrogen transportthrough the substrate and could cause concentration polarization due toinefficient removal of the retentate from the pores.

An important feature of the present invention is that with the thermalmethod, it is possible to provide two different materials that havecoefficients of expansion which are relatively close, it being preferredthat the coefficience for the expansion of the membrane be less thanabout 10% different than for the substrate. Preferably, the ceramicsubstrate and the ceramic oxide portion of the membrane are the same orsubstantially the same. While there are a variety of metals which areuseful as the metal oxide powder part of the hydrogen permeable membranesuch as Ni, Pd, Pd alloys, Nb, Ta, Zr, V or various mixtures thereof,the preferred metal oxides are NiO or the oxides of Pd or a Pd—Ag alloyor mixture thereof. Moreover, the ceramic oxide may be a variety ofmaterials, for instance yttria stabilized zirconia, shrinkable alumina,suitably doped cerates, titanate or zirconates of barium or strontiumand mixtures thereof but the preferred oxide is the yttria stabilizedzirconia or suitably doped barium-cerium-yttrium oxide. Where alumina isused, the alumina should be capable of shrinking upon sintering in airand to this end, an alumina with about 10% water has been foundparticularly satisfactory.

It has been shown that the method of producing hydrogen transfermembranes according to the inventive method produces superior hydrogentransport where the metal powder in the two-part membrane is present inthe range from about 20 to about 80% by volume. Obviously, the greaterthe hydrogen concentration gradient across the membrane the moreimproved the flux will be for a given temperature pressure and otherconditions.

Also as disclosed in the companion application, improved oxygenpermeable membranes have been prepared utilizing those compoundspreviously identified as water splitting compounds in U.S. Pat. No.6,726,893 issued Apr. 27, 2004, the entire disclosure of which is hereinincorporated by reference. The oxygen transfer membranes operate bywater splitting when water comes in contact with the surface of themembrane and is disassociated into hydrogen atoms and oxygen atoms withthe oxygen atoms passing through the membrane leaving the hydrogen atomson the original side. As is well known, thermodynamics insures thatwater splitting continues under these circumstances, thereby increasinghydrogen concentration. Both one part or two part membranes are capableof being manufactured by the method herein described. For one-partmembrane, the oxygen permeable composition is one or a mixture ofSr(Fe_(1-y)Co_(y))O_(x) or Sr(Fe1-yTi_(y))O_(x). The oxygen permeablecomposition may also be formed of a two part membrane in which a metaloxide powder part is selected from Ni, Ag, or Fe or alloys or mixturesthereof with Ni oxide being preferred while the ceramic oxide part maybe selected from CeO₂ doped with lower valence atoms, Gd being preferredor Zr being suitably doped with a lower valence atoms usually from thelanthanides, or a SrFeCo_(0.5)O_(x) or various mixtures thereof. Dopingof these materials is within the skill of the art. These membranes canbe made very thin, also less than about 20 microns and preferably lessthan about 10 microns and exhibit good if not superior oxygenpermeability.

In the inventive thermal method, the metal oxide at the surface of thecermet is reduced initially to form a dense hydrogen permeable portion(or oxygen permeable portion for water splitting), which becomes thickerupon variation in reduction conditions or extended time in the reducingatmosphere. When a hydrogen permeable membrane is in use, and is exposedto a hydrogen containing atmosphere for a long period of time, metaloxide in the interior of the membrane converts by reduction to metal,but due to the use, preferably but not necessarily, of pore formerssufficient porosity remains for the construction to be permeable.

Pore formers which are acceptable are any material which vaporizes attemperatures below the sintering temperature, usually up to about 1000°C. Preferred pore formers are graphite or organic grains such as rice,but corn starch, glass beads, etc. are acceptable. Reducing atmospherescan be a wide variety and composition of gases, but for simplicity,hydrogen is preferred, and if a diluent is used, nitrogen is preferred.

The oxide (metal and ceramic) powders preferably have average diametersin the range of from about 0.1 to about 5 microns and most preferablyabout 0.1 to about 1 micron. The metal portion of the construction ortwo part membrane should be denser than the non-metallic portion of theconstruction or two part membrane after sintering, at least about 96% oftheoretical density and most preferably at least 98%. While all of theceramic oxides in Table 1 are acceptable, the preferred are BCY or YSZ.

While the invention has been particularly shown and described withreference to a preferred embodiment hereof, it will be understood bythose skilled in the art that several changes in form and detail may bemade without departing from the spirit and scope of the invention.

1. A hydrogen permeable membrane on a porous support prepared by: (a)forming a mixture of metal oxide powder and ceramic oxide powder andoptionally a pore former; (b) pressing the mixture to form an article;(c) pre-sintering the article at an elevated temperature in air toremove at least some of the pore former if present and to impartmechanical integrity and porosity to the article; and (d) thereaftersintering the article in a reducing atmosphere to reduce metal oxidenear the surface of the article to a metal and leave metal oxide in thecore of the article, and thereby provide a dense hydrogen permeableportion near the surface of the sintered mixture and a porous corehaving a lower density relative to the dense surface layer; wherein thedense hydrogen permeable portion has a higher initial concentration ofmetal than the remainder of the sintered mixture, the metal oxide isselected from the oxides of Ni, Pd, Pd alloys, Nb, Ta, Zr, V or mixturesthereof, the ceramic oxide is selected from yttria stabilized zirconia,shrinkable alumina, Y or Gd doped cerates, titanates, zirconates ofbarium or strontium or mixtures thereof, and the dense hydrogenpermeable portion has a metal concentration in the range of from about20 to about 80 percent by volume.
 2. The membrane of claim 1, whereinthe pore former is selected from graphite, corn starch, organic grains,glass beads or mixtures thereof.
 3. The membrane of claim 1, wherein thereducing atmosphere includes hydrogen.
 4. The membrane of claim 3,wherein the reducing atmosphere includes nitrogen.
 5. The membrane ofclaim 1, wherein the average diameters of the powders are in the rangeof from about 0.1 to about 5 microns.
 6. The membrane of claim 5,wherein the average diameters of the powders are in the range of fromabout 0.1 to about 1 micron.
 7. The membrane of claim 1, wherein thedense hydrogen permeable portion of composition is at least about 96% oftheoretical density.
 8. The membrane of claim 1, wherein the metal oxidepowder is selected from the oxides of Ni, Pd, Pd alloys, and mixturesthereof.
 9. The membrane of claim 1, wherein the ceramic oxide powder isselected from yttria stabilized zirconia, Y or Gd doped cerates ofbarium or strontium or mixtures thereof.
 10. The membrane of claim 1,wherein the metal oxide powder is selected from the oxides of Ni, Pd, Pdalloys and mixtures thereof and the ceramic oxide powder is selectedfrom yttria stabilized zirconia, Y or Gd doped cerates of barium orstrontium and mixtures thereof.
 11. The membrane of claim 1, wherein thecoefficients of thermal expansion of the dense hydrogen permeableportion and the remainder of the composition are within about 10% ofeach other.
 12. The membrane of claim 1, wherein the metal is Ni or Pdand the ceramic oxide is a suitably doped-barium-cerium-yttrium oxide(BCY) or yttria stabilized zirconia (YSZ).
 13. The membrane of claim 1wherein the article is pre-sintered at a temperature of about 700° C.14. A hydrogen permeable membrane on a porous support prepared by: (a)forming a mixture of metal oxide powder and ceramic oxide powder and apore former; (b) pressing the mixture to form an article, (c)pre-sintering the article at about 700° C. in air to remove at leastsome of the pore former and to impart mechanical integrity and porosityto the article; and (d) thereafter sintering the article in a reducingatmosphere comprising about 200 ppm to about 4% hydrogen to reduce metaloxide near the surface of the article to a metal and leave metal oxidein the core of the article, and thereby provide a dense hydrogenpermeable portion not greater than about 50 microns in thickness and atleast about 96% of theoretical density near the surface of the sinteredmixture and a porous core having a lower density relative to the densesurface layer; wherein the dense hydrogen permeable portion has a higherinitial concentration of metal than the remainder of the sinteredmixture, the metal oxide is selected from the oxides of Ni, Pd, Pdalloys, Nb, Ta, Zr, V or mixtures thereof, the ceramic oxide is selectedfrom yttria stabilized zirconia, shrinkable alumina, Y or Gd dopedcerates, titanates, zirconates of barium or strontium or mixturesthereof, and the dense hydrogen permeable portion has a metalconcentration in the range of from about 20 to about 80 percent byvolume.
 15. The membrane of claim 14, wherein the sintering is performedat a temperature greater than about 1000° C.
 16. The membrane of claim15, wherein at least some of the sintering is performed at a temperaturegreater than about 1400° C.
 17. The membrane of claim 14, wherein thepore former is vaporizable at a temperature of less than about 1000° C.18. The membrane of claim 18, wherein the metal is Ni or Pd and theceramic oxide is suitably doped BCY or YSZ.
 19. The membrane of claim18, wherein the dense hydrogen permeable portion is at least 96% oftheoretical density.
 20. A hydrogen permeable membrane on a poroussubstrate comprising a dense hydrogen permeable metal surface layer notgreater than about 50 microns in thickness and at least about 96% oftheoretical density and a porous metal core having a lower densityrelative to the dense surface layer; wherein the dense hydrogenpermeable portion has a higher initial concentration of metal than theremainder of the sintered mixture, the metal oxide is selected from theoxides of Ni, Pd, Pd alloys, Nb, Ta, Zr, V or mixtures thereof, theceramic oxide is selected from yttria stabilized zirconia, shrinkablealumina, Y or Gd doped cerates, titanates, zirconates of barium orstrontium or mixtures thereof, and the dense hydrogen permeable portionhas a metal concentration in the range of from about 20 to about 80percent by volume.